Polyimide resins are useful per se for molding, in high temperature-resistant adhesive and in laminates. They are also useful in lacquers so as to form, when the solvent is removed, flexible coatings which are strongly adherent to metal substrates and which exhibit excellent resistance to elevated temperatures. They find use in aircraft, in mold and laminated advanced composites, in wire coatings and in printed circuit boards.
Conventional polyimides are generally prepared by the reaction of a dianhydride and a diamine, e.g., pyromellitic dianhydride and diaminodiphenyl ether. State of the art polyimides are prepared by reacting maleic anhydride with organic polyamines, e.g., diaminodiphenyl sulfone or diaminodiphenylmethane. Representative of relevant disclosures of such processes are Sauers, U.S. Pat. No. 3,018,290, Vincent, et al., U.S. Pat. No. 3,625,912 and Crivello, et al., 3,732,189. In general, all such procedures require at least two stages for synthesis, and also one or more purification steps to produce the products in pure form. Moreover, there are some limits as to ultimate physical properties, especially toughness, provided by presently known polyimides.
It has now been discovered, and is the subject of the present invention, that modifying the general procedure somewhat drastically will produce a new class of bisimide resins by a simple one step synthesis and that they will be produced in substantially pure form. The substitution of an unsaturated imide for the commonly employed anhydride and the use of formaldehyde or a formaldehyde precursor to form a methylene bridge between the imide nitrogen and the polyamine nitrogen are the bases of the present invention.